Most people that enjoy working with outdoor bonsai, especially conifers have heard about mycorrhiza. But I have noticed that most are somewhat hesitant as to what it really is what good it does and if it can be combined with bonsai. Since about 95% of all plants on earth live with mycorrhiza and I think it is really important for bonsai, I will try to sort things out.
The word mycorrhiza can be translated with "symbiosis between roots and fungus mycelia". Most of us remember from school that the mushrooms have an extensive "root system" down in the ground. That root system is the real fungus that sometimes sends up fruitbody to the surface with the purpose of spreading spoors, i.e. mushrooms that we happily collect to eat.
The incredibly thin root threads, less then a thousand of an inch, are called hyphae. These hyphae create an expansive network that is called mycelia. Those species of fungus that live in symbiosis with roots are called mycorrhizal fungus. The symbiosis can be summarized by the fact that the tree supplies carbohydrates to the fungus and get water and nutrients back.
The hyphae are capable to enter between the outer cells of the root, which is where the exchange of substances between the fungus and tree take place. This is illustrated by the mycologist Feustel, 1977.
The mycorrhiza also protects the roots against many pathogens. In part the mechanical barrier of the mantle around the rot tip does this, but incredibly enough, the mycelia also create protective antibiotic compounds. The hyphae also emit substances that promote many beneficial soil bacteria that also serve to protect the roots from decease. Some mycorrhizal fungi also produce growth hormones that stimulate the general growth of roots.
Many well-documented experiments have shown the importance of mycorrhiza in nature. An interesting case was the attempts by the authorities in Puerto Rico to introduce pine to the island in the thirties. The mycorrhizal fungus associated with pine was missing and the seeds sprouted and developed well to a height of 4 in. where the growth stopped, the plants slowly withered and died. Various kinds of nutrients were tried without much improvement. In 1955, soil from a pinewood in North Carolina was transported and integrated around 32 struggling one-year-old plants. One year later, those same plants were 6 ft tall. Today there are nice pine forests on Puerto Rico.
Mycorrhiza is especially important under marginal growth conditions and regardless how hard we try, a small pot is not ideal for a tree. Compared to good root conditions in nature we offer a small volume for the roots, violent fluctuations in temperature, available nutrients and water. Exactly in those conditions, mycorrhiza helps as a buffer and equalizer.
There is no need to spend time on the topic whether or not mycorrhiza can grow and thrive in a bonsai pot. All photos in this article are of my own trees and show developed fruitbodies together with spruce, larch, pine and oak. It is not complicated as long as the soil is well draining and not kept too wet. You should also avoid overfeeding with nitrogen.
At the time of repotting, remnants of old soil with mycorrhiza should be included. It is usually easy to see if you did have mycorrhiza in the pot or not as the mycelia forms a spider web like white floss on the surface of the root ball. In transplanting of a plant that one suspect does not have mycorrhiza, it is easy to inoculate it by the introduction of soil from another plant. Try to put the mycorrhiza in direct or close contact with growing white tips of roots.
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© Stefan Strand. This page was updated 27 May 2000